The soil microbe Bacillus thuringiensis (B.t.) is a Gram-positive, spore-forming bacterium characterized by parasporal crystalline protein inclusions. These inclusions often appear microscopically as distinctively shaped crystals. The proteins can be highly toxic to pests and specific in their toxic activity. Certain B.t. toxin genes have been isolated and sequenced, and recombinant DNA-based B.t. products have been produced and approved for use. In addition, with the use of genetic engineering techniques, new approaches for delivering these B.t. endotoxins to agricultural environments are under development, including the use of plants genetically engineered with endotoxin genes for insect resistance and the use of stabilized intact microbial cells as B.t. endotoxin delivery vehicles (Gaertner, F. H., L. Kim 1988! TIBTECH 6:S4-S7). Thus, isolated B.t. endotoxin genes are becoming commercially valuable.
Until the last ten years, commercial use of B.t. pesticides has been largely restricted to a narrow range of lepidopteran (caterpillar) pests. Preparations of the spores and crystals of B. thuringiensis subsp. kurstaki have been used for many years as commercial insecticides for lepidopteran pests. For example, B. thuringiensis var. kurstaki HD-1 produces a crystalline .delta.-endotoxin which is toxic to the larvae of a number of lepidopteran insects.
In recent years, however, investigators have discovered B.t. pesticides with specificities for a much broader range of pests. For example, other species of B.t., namely israelensis and tenebrionis (a.k.a. B.t. M-7, a.k.a. B.t. san diego), have been used commercially to control insects of the orders Diptera and Coleoptera, respectively (Gaertner, F. H. 1989! "Cellular Delivery Systems for Insecticidal Proteins: Living and Non-Living Microorganisms," in Controlled Delivery of Crop Protection Agents, R. M. Wilkins, ed., Taylor and Francis, New York and London, 1990, pp. 245-255). See also Couch, T. L. (1980) "Mosquito Pathogenicity of Bacillus thuringiensis var. israelensis," Developments in Industrial Microbiology 22:61-76; Beegle, C. C., (1978) "Use of Entomogenous Bacteria in Agroecosystems," Developments in Industrial Microbiology 20:97-104. Krieg, A., A. M. Huger, G. A. Langenbruch, W. Schnetter (1983) Z. ang. Ent. 96:500-508, describe Bacillus thuringiensis var. tenebrionis, which is reportedly active against two beetles in the order Coleoptera. These are the Colorado potato beetle, Leptinotarsa decemlineata, and Agelastica alni.
Recently, new subspecies of B.t. have been identified, and genes responsible for encoding active .delta.-endotoxin proteins have been isolated (Hofte, H., H. R. Whiteley 1989! Microbiological Reviews 52(2):242-255). Hofte and Whiteley classified B.t. crystal protein genes into 4 major classes. The classes were CryI (Lepidoptera-specific), CryII (Lepidoptera- and Diptera-specific), CryIII (Coleoptera-specific), and CryIV (Diptera-specific). The discovery of strains specifically toxic to other pests has been reported. (Feitelson, J. S., J. Payne, L. Kim 1992! Bio/Technology 10:271-275).
The cloning and expression of a B.t. crystal protein gene in Escherichia coli has been described in the published literature (Schnepf, H. E., H. R. Whiteley 1981! Proc. Natl. Acad. Sci. USA 78:2893-2897). U.S. Pat. No. 4,448,885 and U.S. Pat. No. 4,467,036 both disclose the expression of B.t. crystal protein in E. coli. Hybrid B.t. crystal proteins have been constructed that exhibit increased toxicity and display an expanded host range to a target pest. See U.S. Pat. Nos. 5,128,130 and 5,055,294. U.S. Pat. Nos. 4,797,276 and 4,853,331 disclose B. thuringiensis strain tenebrionis (a.k.a. M-7, a.k.a. B.t. san diego) which can be used to control coleopteran pests in various environments. U.S. Pat. No. 4,918,006 discloses B.t. toxins having activity against dipterans. U.S. Pat. No. 4,849,217 discloses B.t. isolates which have activity against the alfalfa weevil. U.S. Pat. No. 5,208,017 discloses coleopteran-active Bacillus thuringiensis isolates. U.S. Pat. No. 5,151,363 and U.S. Pat. No. 4,948,734 disclose certain isolates of B.t. which have activity against nematodes. As a result of extensive research and investment of resources, other patents have issued for new B.t. isolates and new uses of B.t. isolates. However, the discovery of new B.t. isolates and new uses of known B.t. isolates remains an empirical, unpredictable art.
A majority of Bacillus thuringiensis .delta.-endotoxin crystal protein molecules are composed of two functional segments. The protease-resistant core toxin is the first segment and corresponds to about the first half of the protein molecule. The three-dimensional structure of a core segment of a cryIIIA B.t. .delta.-endotoxin is known and it is proposed that all related toxins have that same overall structure (Li, J., J. Carroll, D. J. Ellar 1991! Nature 353:815-821). The second half of the molecule is the second segment. For purposes of this application, this second segment will be referred to herein as the "protoxin segment." The protoxin segment is believed to participate in toxin crystal formation (Arvidson, H., P. E. Dunn, S. Strand, A. I. Aronson 1989! Molecular Microbiology 3:1533-1534; Choma, C. T., W. K. Surewicz, P. R. Carey, M. Pozsgay, T. Raynor, H. Kaplan 1990! Eur. J. Biochem. 189:523-527). The full toxin molecule is rapidly processed to the resistant core segment by protease in the insect gut. The protoxin segment may thus convey a partial insect specificity for the toxin by limiting the accessibility of the core to the insect by reducing the protease processing of the toxin molecule (Haider, M. Z., B. H. Knowles, D. J. Ellar 1986! Eur. J. Biochem. 156:531-540) or by reducing toxin solubility (Aronson, A. I., E. S. Han, W. McGaughey, D. Johnson 1991! Appl. Environ. Microbiol. 57:981-986).
Chimeric proteins joined within the toxin domains have been reported between CryIC and CryIA(b) (Honee, G., D. Convents, J. Van Rie, S. Jansens, M. Perferoen, B. Visser 1991! Mol. Microbiol. 5:2799-2806); however, the activity of these chimeric proteins was either much less, or undetectable, when compared to CryIC on a relevant insect.
Honee et al. (Honee, G., W. Vriezen, B. Visser 1990! Appl. Environ. Microbiol. 56:823-825) also reported making a chimeric fusion protein by linking tandem toxin domains of CryIC and CryIA(b). The resulting protein had an increased spectrum of activity equivalent to the combined activities of the individual toxins; however, the activity of the chimeric was not increased toward any one of the target insects.